It is known to apply pressure pulses to molding material, for example molding sand, in a mold form in which a model or pattern of the casting to be made has been placed. This air pulse method is suitable, basically, for foundry work. If the model is complex and has shapes which are difficult to reproduce in the molding sand, homogeneous compaction is necessary. The compaction must be tight so that a hard replica of the model is obtained, even if the model is subject to abrupt differences in dimensions, typically levels, or has contours which differ only slightly from the edge of the model.
U.S. Pat. No. 4,592,406, Damm, assigned to the assignee of the present application, describes an arrangement in which a gas pervious layer is located above the surface of the molding material The gas permeability of this layer is less in the region above the model or pattern than in the region adjacent the edges thereof, that is, the regions approaching the molding form or flask, and where the model is no longer located. This arrangement provides for a substantially improved matching of the hardness of the form to be obtained to the shape of the model.
German Patent Disclosure Document DE OS 37 40 775, Fischer, describes a process in which compaction is carried out by a plurality of sequential pressure pulses, in which the pressure gradient of the first pressure pulse is less than the pressure gradient of the second or subsequent pressure pulse. This dual pressure pulse increases the repetition time or cadence of a foundry form making machine.